Many older adults are less steady when exerting low forces and lifting light loads. The greater fluctuations in force exhibited by older adults can be caused either by increased discharge rate variability or by altered amounts of correlated activity among the motor units. Based on prior experimental and computational studies, we hypothesize that the greater fluctuations experienced by older adults during voluntary contractions are due to direct and indirect effects of correlated motor unit discharge. Aim 1 will extend our current model of the electromyogram (EMG) to simulate the reorganization of the neuromuscular system that occurs with advancing age. The simulations will assess the effect of cancellation of motor unit potentials on the amplitude of the surface EMG and will evaluate the validity of estimating common modulation of motor unit discharge from the EMG signal. Aim 2 will evaluate the fluctuations in force and acceleration during isometric and anisometric contractions performed with the first dorsal interosseus muscle by young and old adults. The time- and frequency-domain characteristics will be compared with prior experimental measurements and modeling results for young adults. Aim 3 will compare the contributions of the two candidate neural mechanisms (discharge rate variability and correlated discharge) to the differences between young and old adults in index finger acceleration when performing isometric and anisometric contractions with the first dorsal interosseus muscle. Aim 4 will examine the role of coactivation at the level of the motor unit by measuring the amount and timing of correlated discharge between motor units in the agonist and antagonist muscles. These studies will provide novel information on the neural mechanisms that contribute to the reduced ability of older adults to perform steady contractions.